The efficacy of crop fertilizers depends on their placement relative to the crop seed because the nutrient requirements of crops change over the crop life cycle. During their sprouting and early growth stages, crops take in a significant amount of phosphorus which promotes rapid growth and early maturity. Accordingly, phosphorus-rich fertilizer (typically available in liquid form) should be placed in the immediate vicinity of the seeds, where it can be absorbed by the nascent root systems. During their early growth stages until their maturity, crops take in nitrogen which stimulates stem and leaf development. Accordingly, nitrogen-rich fertilizer (typically available either in liquid or granular form) should be placed in a zone below and surrounding the seeds, where it can be accessed by the root systems as they develop downwards and outwards from the seeds. Moreover, mixing of phosphorus-rich fertilizer and nitrogen-rich fertilizer in the soil should be avoided because such mixing can have toxicity effects known as “seed burn”.
Air seeders are commonly used in modern farming practice to simultaneously place granular crop seeds (e.g., cereal grain seeds) and fertilizer below the soil surface. In general, an air seeder comprises a mobile frame supporting a seed hopper, a fertilizer hopper, a seed metering system, a fertilizer metering system, a seed manifold, a fertilizer manifold, a pneumatic seed delivery system, a fertilizer delivery system, and a plurality of soil openers each of which has at least one seed passage and one fertilizer passage. A tractor tows the mobile frame across the soil surface causing the soil openers to cut furrows into the soil surface. Simultaneously, the seed metering system feeds a seed stream from the seed hopper into the seed manifold. The pneumatic seed delivery system creates an airstream that evenly distributes the seeds stream to the seed passages, which deposit the seeds in the furrows. The fertilizer metering system, fertilizer hopper, fertilizer delivery system and fertilizer passages function in an analogous manner with respect to the fertilizer, although the fertilizer delivery system may be adapted to deliver liquid or gaseous fertilizers.
In the prior art, “double-shoot” soil openers are designed to deposit crop seeds in paired rows with a band of fertilizer placed in between and slightly below the level of the seed rows. Such soil openers have a seed distributor that divides the seed stream into two portions that are deposited in laterally spaced apart rows. A small protrusion on the underside of the soil opener cuts a small trench in between the two seed rows and slightly deeper than the furrow. The fertilizer passage is positioned to deposit the fertilizer into the trench. However, use of such soil openers to deposit a mixture of phosphorus-rich and nitrogen-rich fertilizers will result in suboptimal placement of the phosphorus-rich fertilizer away from the seeds and mixing of the fertilizers that will increase the risk of seed burn.